The present invention relates microelectronic devices and more particularly to edge termination for silicon carbide Schottky devices.
High voltage silicon carbide (SiC) Schottky diodes, which can handle voltages between 600 V and 2.5 kV, are expected to compete with silicon PIN diodes fabricated of similar voltage ratings. Such diodes may handle as much as 100 amps of current, depending on their active area. High voltage Schottky diodes have a number of important applications, particularly in the field of power conditioning, distribution and control.
An important characteristic of a SiC Schottky diode in such applications is its switching speed. Silicon-based PIN devices typically exhibit relatively poor switching speeds. A silicon PIN diode may have a maximum switching speed of approximately 20 kHz, depending on its voltage rating. In contrast, silicon carbide-based devices are theoretically capable of much higher switching speeds, for example, in excess of 100 times better than silicon. In addition, silicon carbide devices may be capable of handling a higher current density than silicon devices.
However, reliable fabrication of silicon carbide-based Schottky devices may be difficult. Typical edge termination in SiC Schottky diodes require ion implantation of p-type dopants into the crystal. Such implants may cause substantial damage to the crystal lattice, which may require high temperature annealing to repair such defects. This high-temperature anneal step ( greater than 1500xc2x0 C.) may be undesirable for a number of reasons. For example, high temperature anneals tend to degrade the surface of SiC on which the Schottky contact is to be made, as silicon tends to dissociate from exposed surfaces of the crystal under such a high-temperature anneal. Loss of silicon in this manner may result in a reduced quality Schottky contact between metal and the semiconductor surface. High temperature anneals have other drawbacks as well. Namely, they are typically time-consuming and expensive. Moreover, implantation of p-type (Al) dopants may cause substantial lattice damage, while other species (B) have poor activation rates.
A conventional SiC Schottky diode structure has an n-type SiC substrate on which an nxe2x88x92 epitaxial layer, which functions as a drift region, is formed. The device typically includes a Schottky contact formed directly on the nxe2x88x92 layer. Surrounding the Schottky contact is a p-type JTE (junction termination extension) region which is typically formed by ion implantation. The implants may be aluminum, boron, or any other suitable p-type dopant. The purpose of the JTE region is to prevent the electric field crowding at the edges, and to prevent the depletion region from interacting with the surface of the device. Surface effects may cause the depletion region to spread unevenly, which may adversely affect the breakdown voltage of the device. Other termination techniques include guard rings and floating field rings which are more strongly influenced by surface effects. A channel stop region may also be formed by implantation of n-type dopants such as Nitrogen or Phosphorus in order to prevent the depletion region from extending to the edge of the device.
Additional conventional termination of SiC Schottky diodes are described in xe2x80x9cPlanar Terminations in 4H-SiC Schottky Diodes With Low Leakage And High Yieldsxe2x80x9d by Singh et al., ISPSD ""97, pp. 157-160. A p-type epitaxy guard ring termination for a SiC Schottky Barrier Diode is described in xe2x80x9cThe Guard-Ring Termination for High-Voltage SiC Schottky Barrier Diodesxe2x80x9d by Ueno et al., IEEE Electron Device Letters, Vol. 16, No. 7, July, 1995, pp. 331-332. Additionally, other termination techniques are described in published PCT Application No. WO 97/08754 entitled xe2x80x9cSiC Semiconductor Device Comprising A PN Junction With A Voltage Absorbing Edge.xe2x80x9d
Embodiments of the present invention may provide a silicon carbide Schottky rectifier having a silicon carbide voltage blocking layer having a predefined surface doping level and a Schottky contact on the silicon carbide voltage blocking layer. A silicon carbide epitaxial region is also provided on the silicon carbide voltage blocking layer and adjacent the Schottky contact. The silicon carbide epitaxial region has a thickness and a doping level designed to provide a selected charge per unit area in the silicon carbide epitaxial region. The charge per unit area in the silicon carbide epitaxial regions, also referred to as the junction termination extension (JTE) charge, is selected based on the surface doping of the blocking layer. In particular embodiments, the JTE charge is greater than 50% of an optimal JTE charge as determined by the surface doping of the blocking layer. Furthermore, it is preferred that the JTE charge is not greater than the optimal charge value.
In further embodiments of the present invention, a silicon carbide Schottky rectifier is provided having a silicon carbide voltage blocking layer and a Schottky contact on the silicon carbide voltage blocking layer. A silicon carbide epitaxial termination region is provided on the voltage blocking layer and adjacent the Schottky contact. The product of the thickness and doping concentration of the silicon carbide epitaxial region is greater than about 50% of             (                        ϵ          r                xc3x97                  ϵ          0                xc3x97                  E          C                    )        q    ;
where:
xcex5r is the relative dielectric constant of SiC;
xcex50 is dielectric constant of air;
EC is the critical electric field of SiC; and
q is the electronic charge.
In further embodiments, the product of the thickness and doping concentration are not greater than about 100% of             (                        ϵ          r                xc3x97                  ϵ          0                xc3x97                  E          C                    )        q    .
In still further embodiments, the thickness and doping concentration are not less than about 75% of             (                        ϵ          r                xc3x97                  ϵ          0                xc3x97                  E          C                    )        q    .
In still further embodiments of the present invention, the silicon carbide epitaxial region extends from the Schottky contact from about 1.5 to about 5 times the thickness of the blocking layer. Additionally, a non-ohmic contact may be provided between the silicon carbide epitaxial termination region and the Schottky contact.
In embodiments of the present invention where the silicon carbide epitaxial region has a first conductivity type and the voltage blocking layer has a second conductivity type opposite the first conductivity type, the edge termination may also include a region of first conductivity type silicon carbide in the voltage blocking layer having a carrier concentration higher than that of the voltage blocking layer and adjacent a periphery of the silicon carbide epitaxial region opposite the Schottky contact.
In additional embodiments of the present invention, the Schottky rectifier may also include a first layer of silicon carbide of a first conductivity type the same as a conductivity type of the blocking layer and disposed between the blocking layer and a silicon carbide substrate. The first layer of silicon carbide may have a carrier concentration higher than the blocking layer. A second layer of silicon carbide of the first conductivity type may also be provided on the substrate opposite the first layer of silicon carbide so as to provide a layer of silicon carbide having a carrier concentration higher than a carrier concentration of the substrate. An ohmic contact may be provided on the second layer of silicon carbide. In such embodiments, the second layer may be an implanted layer of first conductivity type silicon carbide. Furthermore, the silicon carbide epitaxial region may be of a second conductivity type opposite that of the first conductivity type. In particular, the first conductivity type may be n-type and the second conductivity type may be p-type.
In other embodiments of the present invention, a Schottky rectifier is provided which includes an n-type silicon carbide substrate, an n-type silicon carbide blocking layer on the silicon carbide substrate, a Schottky contact on the silicon carbide blocking layer, an epitaxial region of p-type silicon carbide on the silicon carbide blocking layer and adjacent the Schottky contact so as to form a non-ohmic contact between the p-type epitaxial region and the Schottky contact, and an ohmic contact on the substrate opposite the blocking layer.
In further embodiments of the present invention, a plurality of p-type silicon carbide islands on the blocking layer may be provided. In such embodiments, the Schottky contact overlaps the plurality of p-type islands.
In yet other embodiments of the present invention, a Schottky rectifier is provided which includes an n-type silicon carbide substrate, an n-type silicon carbide blocking layer on the silicon carbide substrate, a Schottky contact on the silicon carbide blocking layer, an epitaxial region of p-type silicon carbide on the silicon carbide blocking layer and adjacent the Schottky contact so as to form a non-ohmic contact between the p-type epitaxial region and the Schottky contact and an ohmic contact on the substrate opposite the blocking layer. The p-type silicon carbide epitaxial region extends from the Schottky from contact about 2 to about 3 times the thickness of the blocking layer. Such embodiments may also include a plurality of p-type silicon carbide islands on the blocking layer. If so, then the Schottky contact overlaps the plurality of p-type islands.
In method embodiments of the present invention, a method of fabricating a termination region of a silicon carbide Schottky rectifier includes forming an epitaxial layer of p-type silicon carbide having a thickness and a doping level selected so as to provide a charge in a region of p-type silicon carbide adjacent to a Schottky contact of the Schottky rectifier proportional to the surface doping of a silicon carbide blocking layer of the Schottky rectifier and patterning the epitaxial layer of p-type silicon carbide so as to provide the region of p-type silicon carbide adjacent to the Schottky contact of the Schottky rectifier.
In such method embodiments, the patterning the epitaxial layer of p-type silicon carbide may be provided by forming an oxide layer on the p-type epitaxial layer of silicon carbide, patterning the oxide layer so as to expose a portion of the p-type epitaxial layer corresponding to the Schottky contact, thermally oxidizing the exposed portion of the p-type epitaxial layer at a temperature and for a time sufficient to consume all of the expose portion of the p-type epitaxial layer, and etching the thermally oxidized portion of the p-type epitaxial layer so as to expose a portion of the blocking layer on which the Schottky contact is formed. Furthermore, thermally oxidizing the p-type epitaxial layer may be carried out in a dry ambient environment. Additionally, thermally oxidizing the p-type epitaxial layer may be accomplished by oxidizing the p-type epitaxial layer at a temperature of less than about 1300xc2x0 C.
In further embodiments, etching the thermally oxidized portion of the p-type epitaxial layer may be preceded depositing an ohmic metal on a surface of a silicon carbide substrate opposite the blocking layer and annealing the ohmic metal so as to form an ohmic contact to the silicon carbide substrate. Such a deposition of an ohmic metal may also be preceded by implanting n-type dopants in the substrate so as to form a region of n-type silicon carbide having a carrier concentration higher than a carrier concentration of the substrate. In such embodiments, the ohmic metal may be deposited on the implanted region of the substrate. Furthermore, the implantation of n-type dopants may be carried out prior to thermally oxidizing the exposed portion of the p-type epitaxial layer such that thermally oxidizing the exposed portion of the p-type epitaxial layer also activates the n-type dopants.
In yet other embodiments of the present invention, etching the thermally oxidized portion of the p-type epitaxial layer is followed by depositing a Schottky metal on the exposed portion of the blocking layer.
In still other embodiments of the present invention, the good quality passivating oxide layer is patterned so as to expose a portion of the p-type epitaxial layer corresponding to the Schottky contact including a plurality of regions of exposed portions of the p-type epitaxial layer. The exposed portion of the p-type epitaxial layer including the plurality of regions is thermally oxidized at a temperature and for a time sufficient to consume all of the exposed portion of the p-type epitaxial layer. The thermally oxidized portion of the p-type epitaxial layer is etched so as to expose a portion of the blocking layer on which the Schottky contact is formed thereby providing a plurality of exposed portions of the blocking layer spaced apart by islands of p-type silicon carbide.
In additional embodiments of the present invention, patterning the oxide layer so as to expose a portion of the p-type epitaxial layer corresponding to the Schottky contact is preceded by patterning the oxide layer and the p-type epitaxial layer to expose a portion of the blocking layer at the periphery of the Schottky rectifier, and implanting n-type dopants in the exposed portion of the n-type blocking layer so as to provide a region of implanted n-type dopants adjacent the periphery of the p-type epitaxial layer.
In particular embodiments, the thickness and doping level of the silicon carbide epitaxial region are selected so as to provide a charge in the region of p-type silicon carbide adjacent to a Schottky contact of the Schottky rectifier which is from about 50% to about 100% of             Q              JTE        ,        opt              =                  (                              ϵ            r                    xc3x97                      ϵ            0                    xc3x97                      E            C                          )            q        ;
where:
QjTE,opt is the optimum JTE charge;
xcex5r is the relative dielectric constant of SiC;
xcex50 is dielectric const. of air;
EC is the critical electric field of SiC; and
q is the electronic charge.
Furthermore, the thickness and doping level may be about 75% of       Q          JTE      ,      opt        =                    (                              ϵ            r                    xc3x97                      ϵ            0                    xc3x97                      E            C                          )            q        .  
In further method embodiments of the present invention, a method of fabricating a silicon carbide Schottky rectifier is provided by forming a first n-type silicon carbide epitaxial layer on an n-type silicon carbide substrate, wherein the first n-type silicon carbide epitaxial layer has a carrier concentration less than a carrier concentration of the silicon carbide substrate. A p-type silicon carbide epitaxial layer is formed on the first n-type epitaxial layer. A passivating oxide is deposited on the p-type silicon carbide epitaxial layer and the passivating oxide patterned to expose a portion of the p-type silicon carbide corresponding to a Schottky contact of the Schottky rectifier. The exposed portion of the p-type silicon carbide is thermally oxidized to oxidize the p-type silicon carbide to the first n-type epitaxial layer. An ohmic contact metal is then deposited on the substrate opposite the first n-type epitaxial layer and annealed so as to provide an ohmic contact to the substrate. Then the oxidized p-type silicon carbide is removed to expose a portion of the first n-type epitaxial layer corresponding to the Schottky contact and a Schottky metal deposited on the exposed portion of the first n-type epitaxial layer so as to provide a Schottky contact to the first n-type epitaxial layer.
In further embodiments of the present invention, the p-type silicon carbide epitaxial layer has a thickness and a doping level selected so as to provide a charge in a region of p-type silicon carbide adjacent to the Schottky contact of the Schottky rectifier proportional to the surface doping of the first n-type epitaxial layer.
In other embodiments, the method further includes forming a second layer of n-type silicon carbide, wherein the second layer of n-type silicon carbide is disposed between the silicon carbide substrate and the first n-type epitaxial layer and has a carrier concentration higher than the carrier concentration of the silicon carbide substrate.
In still other embodiments, the method may also include patterning the p-type epitaxial layer to expose a portion of the first n-type epitaxial layer adjacent the periphery of the Schottky rectifier, implanting n-type dopants in the exposed portion of the first n-type epitaxial layer and thermally annealing the implanted dopants so as to activate the dopants. In such embodiments, the deposition of a passivating oxide may also include depositing a passivating oxide on the p-type epitaxial layer and the implanted region of the first n-type epitaxial layer. In particular embodiments, the thermal anneal of the implanted dopants is carried out at a temperature of less than about 1300xc2x0 C.
Furthermore, additional method embodiments may also include implanting n-type dopants in the silicon carbide substrate opposite the first n-type epitaxial layer so as to provide a region of n-type silicon carbide having a carrier concentration higher than the carrier concentration of the silicon carbide substrate. In such embodiments, implanting n-type dopants may be followed by depositing an oxide on the implanted region of the silicon carbide substrate and the thermal oxidation step may be followed by removing the oxide from the implanted region of the silicon carbide substrate.
The advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein: